1. Field of the Invention
The present invention relates to a drive circuit for driving at least one LED (Light Emitting Diode) as an illumination element of a display device used in a portable electronic device and, particularly, to an LED drive circuit for driving at least one LED as an illumination element of a battery driven, LCD (Liquid Crystal Display) device mounted on a portable electronic device such as a portable telephone set of PHS (Personal Handyphone System) or portable telephone system, an electronic book or a PDA. The LED drive circuit is capable of easily driving an LED having high forward voltage drop Vf, is compact and low power consumption, has a small variation of luminance and is capable of restricting change of luminance even when the battery voltage is lowered.
2. Description of Related Art
As a source of illumination light such as a back light or side light for a display device of a battery driven electronic device such as a portable telephone set of such as PHS or portable telephone system or a portable electronic device, LED has been used.
A drive circuit for LED generates a LED drive voltage by boosting a battery voltage by a booster circuit such as a charge pump circuit utilizing a voltage doubler and by regulating the boosted voltage to a constant voltage by means of a voltage regulator.
FIG. 6 is a circuit diagram of one of examples of a conventional LED drive circuit.
In the LED drive circuit shown in FIG. 6, a charge pump circuit 12 adds an output voltage of a lithium battery 11 in a range from 3.0V to 4.2V, for example, 3.6V, to a capacitor C1 charged to the output voltage of the lithium battery 11, by on-off switching the output voltage of the lithium battery 11 at a predetermined frequency. An output voltage of the charge pump circuit 12, which is about 7.2V substantially twice the output voltage of the lithium battery 11, is applied to a capacitor C2 to charge the latter. The voltage of the capacitor C2 is regulated to a constant voltage, for example, 5V, by a voltage regulator 13.
The voltage regulator 13 regulates the voltage of the capacitor C2 by a voltage drop transistor thereof to generate a stabilized voltage of 5V and a power charging capacitor Co is charged by the stable voltage of 5V. A series circuit including a resistor R and an LED 15 is connected between an output terminal 14 of the voltage regulator 13 and ground so that the LED 15 is driven by the stabilized voltage through the resistor R.
In this case, a drive current Io becomes Io=(5xe2x88x92Vf)/R, where R is a resistance value of the resistor R and Vf is a terminal voltage of the LED 15 when it is driven.
In such LED drive circuit, however, luminance of an LED tends to vary. Particularly, the terminal voltage Vf of a white light emitting LED used as a back light of an LCD, a blue light emitting LED or a green light emitting LED is as high as about 3.5V in average and the terminal voltage Vf itself varies. Therefore, luminance of a driven LED may be low or there may be LED which can not emit light.
FIG. 7(a) shows a white LED drive voltage (forward terminal voltage Vf) vs. drive current (forward current Ifp) characteristics of the white LED and FIG. 7(b) shows a relative luminous intensity vs. drive current (forward current Ifp) characteristics thereof.
A variation of the drive voltage vs. drive current characteristics shown by a double arrow in FIG. 7(a) may be restricted by using a variable resistor as the resistor R (FIG. 6) and regulating a resistance value thereof. However, it requires a resistance regulation step, additionally. Further, in a case where the voltage regulator 13 is used, the power charging capacitor Co having capacitance as large as several xcexcFxcx9cten and several xcexcF must be used. Even if a chip capacitor is used as the power capacitor Co, a problem occurs in reducing the size and weight of a portable telephone set of PHS or portable telephone system.
Further, when a voltage regulator utilizing a voltage drop transistor is used, power consumption is increased and, further, a size of the power circuit for a portable electronic device becomes large. In order to avoid these problems, there is an LED drive circuit in which a voltage is boosted by using a switching power circuit.
However, the switching power circuit requires an inductance element, so that electromagnetic noise tends to occur and a countermeasure against such noise must be employed in the switching power circuit itself and/or other circuits. Further, an inductance of a coil used as the inductance element must be 100 xcexcHxcx9cseveral hundreds xcexcH, whose size may be at least 5 mmxc3x975 mm. As a result, the inductance element also becomes problem in reducing the size and weight of a portable telephone set of PHS or portable telephone system.
On the other hand, JP H11-305198A discloses a technique for maintaining a luminance of an LED constant when the LED is used as a back light of an LCD. In this technique, a constant current circuit is provided to drive the LED with a constant current. Further, JU H5-27789A discloses a technique similar to that disclosed in JP H11-305198A. Further, JP H3-129884A discloses a technique in which an LED is used as a direct display element and is driven by a charge pump circuit. However, a constant current driving is not employed in the latter prior art.
It can be said that these LED drive circuit techniques disclosed in the above-mentioned prior arts can not be used in a device having a compact and light-weight casing of such as a portable telephone set. The reason for this is that a reference voltage is generated by a series circuit of a Zener diode and a MOS transistor. Since, in such circuit, an operating voltage of a constant current circuit becomes high and it is impossible to maintain a luminance of emitted light constant in a wide range of an output voltage characteristics of the battery, which shows a gradual reduction of output voltage with use of the battery. Further, since the constant current drive using a bipolar transistor is employed and the driving constant current varies with variation of temperature, luminance of LED varies when temperature of the compact and lightweight casing of the portable telephone set rises. Such variation of luminance of LED may become modest to some extent in the circuit disclosed in JP H11-305198A in which a number of LED""s provided by a plurality of parallel connected series circuits each including four series connected LED""s are driven simultaneously. However, in a low power consumption back light illuminating circuit using 2 to 6 LED""s, such variation of luminance becomes a problem.
In order to elongate a battery life of an electronic device such as a portable telephone set, it is usual that a power consumption of the electronic device is reduced by reducing a display luminance of an LCD thereof in night time relative to luminance in day time. When luminance of LED is regulated in such manner, it is necessary to regulate a drive current of the LED. Therefore, a drive current regulator circuit must be provided separately, causing a circuit size to be increased. The provision of such drive current regulator circuit becomes obstacles in giving multifunction to the electronic device, in reducing the size and weight of the electronic device and in integrating the circuits.
An object of the present invention is to provide a compact LED drive circuit for driving an LED for use to illuminate a display device of a portable electronic device, which is capable of easily driving the LED having high terminal voltage Vf with small power consumption and of restricting a variation of luminance of the LED even when an output voltage of a battery thereof is reduced.
In order to achieve the above object, a drive circuit for an LED for use to illuminate a display device of a portable electronic device according to the present invention is featured by comprising a battery power source, a booster circuit for boosting an output voltage of the battery power source by charging a first capacitor with the output voltage of the battery power source by switching the latter at a predetermined frequency, charging a second capacitor by boosting the voltage of the first capacitor to substantially n/m times by an on-off switching, which is complement to the switching of the output voltage of the battery power source, where n and m are integers equal to or larger than 2, respectively, and n greater than m, at least one LED connected to an output terminal of the booster circuit as a load and a constant current circuit connected to said LED, for driving the LED with a constant current, the constant current circuit including a MOS transistor and means for generating the constant current by controlling the MOS transistor by comparing a predetermined reference voltage with a voltage according to a drive current of the LED.
In the construction of the LED drive circuit mentioned above, the booster circuit having a first and second capacitors and using a n/m voltage type rectifying switching is used and the constant current circuit having the MOS transistor as the output transistor is connected in series with the LED as a load of the booster circuit to drive the LED with constant current. With this circuit construction, there is substantially no electromagnetic noise generated and it is possible to maintain a terminal voltage of the LED substantially constant according to the diode characteristics of the LED even when voltage ripple appears in an output of the booster circuit due to the n/m voltage boost, by setting the switching frequency at high to absorb the ripple component on the side of the constant current circuit. Since the ripple component is absorbed on the side of the constant current circuit, the LED is driven with substantial constant current and constant voltage and variation of luminance of the LED is restricted.
Further, the operating voltage of the constant current circuit having the MOS transistor in its output stage is as low as 1V or less. Therefore, a voltage across a voltage detecting resistor inserted in series therewith can be made several hundreds mV or less, so that it is possible to maintain substantially constant luminance characteristics in a wide range of the voltage drop characteristics of the battery.
As a result, a large power capacitor becomes unnecessary and a voltage regulator is also unnecessary. Therefore, it becomes possible to realize an LED drive circuit, which has a restricted power consumption, restricted luminance variation of LED""s and is suitable for use in a compact device.